A tank-venting system generally has a fuel tank and a tank-venting valve which is connected to the intake pipe of an internal combustion engine so that fuel vapors can be evacuated by suction with the aid of the underpressure in the intake pipe. Usually, the volume located above the fuel in the tank is not directly evacuated by suction; rather, an adsorption filter, usually an activated carbon filter, is connected between the tank and the tank-venting valve. This activated carbon filter adsorbs fuel in those periods in which no drawing out by suction via the intake pipe occurs, for example when the internal combustion engine is at standstill or when the tank-venting valve is kept closed due to the current operating state.
There is a risk of tank-venting systems developing leaks or the tank-venting valve not operating correctly. The operability of systems of this kind is therefore to be checked repeatedly during the operation of a motor vehicle.
The most important method for checking the operability of a motor vehicle tank-venting system is based on a proposal of the California Environmental Authority CARB. According to this method, when opening the tank-venting valve, a check is made as to whether a lambda controller has to carry out a correction of its control output. This is always the case when air with fuel vapor is evacuated by suction of the tank-venting system. However, it is also the case that the adsorption filter can be completely regenerated and that the fuel in the tank is completely evaporated. When the tank-venting valve is opened, no fuel is supplied in addition to that which is supplied to the injection valves of the internal combustion engine according to the control output of the lambda control. In such a case in which therefore no fuel is supplied by the tank-venting system, that is, the lambda controller does not have to carry out a correction, it is unclear whether the tank-venting system has developed a leak or whether no fuel is being supplied because of the mentioned reasons. In order to be able to decide this question, according to the known method, an evaluation of the signal from the lambda controller only occurs when a fuel temperature sensor indicates that a predetermined minimum fuel temperature is exceeded and a tank level sensor indicates that the vehicle tank has been filled. It is assumed that fuel vapor would then have to be present in the system in any event which vapor is drawn in when the tank-venting valve is opened and which then leads to a correction of the lambda controller. However, with this method incorrect decisions repeatedly occur if there is in fact evaporated fuel in the tank, refilling occurs with the same kind of fuel and the adsorption filter is largely regenerated.
Accordingly, there was still the problem of specifying a method for checking the operability of a motor vehicle tank-venting system which emits as few unjustified fault reports as possible. In addition, there was the problem of specifying a tank-venting system whose operability can be checked in a particularly reliable manner.